FIG. 9 is an exploded view of a partially constructed prior art planar carbon segment commutator. The commutator has a copper connector 10′ which is in the form of a disc with radially extending arms. A non-conductive hub 30′, typically of phenolic, is over molded to the copper connector. A carbon disc 20′ is soldered to the copper connector 10′ and then the radial arms are bent into U-shape terminals or tangs for connection of armature lead wires. Then the copper/carbon disc assembly is cut into a plurality of individual commutator segments, held together by the hub. As it is very difficult to solder directly to the carbon disc, the surface of the carbon disc to be soldered to the connector is first electroplated with a layer of nickel 40′ and then a layer of copper 45′ is electroplated to the nickel layer. Usually a layer of solder 50′ is applied to the layer of copper to ensure good adhesion and reliability of the solder connection to the copper connector. Small fingers or anchors are integrally formed on the copper connector to strengthen the fixation of the connector 10′ to the hub 30′.
However, even with the nickel plating 40′ and copper plating 45′, the solder connection between the carbon disc 20′ and the copper connector 10′ is problematic. The bonding force between the carbon disc and the nickel layer is weak and the plating processes are time consuming and expensive. Furthermore, during electroplating, the electroplating solution may penetrate the carbon layer and is difficult to remove. If the electroplating solution is not removed it will erode the coatings thereby reducing electrical conductivity between the carbon disc and the copper connector and reducing the working life of the commutator.
Carbon commutators in which the carbon layer is directly molded to the copper connector are known but in practice the electrical connection between the carbon and the copper has a higher contact resistance than the soldered commutators and requires a thicker layer of carbon to ensure good physical strength. This also added resistance to the current path through the commutator from brush contact surface to tang. For extra low voltage applications and for high current applications this additional resistance is a issue. For high current applications the added resistance results in excessive heating of the commutator.
Hence there is a desire for an improved carbon commutator which can solve the above-mentioned problems.